The prior art discloses the manufacture of non-linear carbonaceous fibers having a reversible deflection ratio of greater than 1.2:1 and derived from polymeric compositions such as polyacrylonitrile (PAN). The polymeric material is spun into fibers and can be collected into multifiber assemblies, such as fiber tows containing more than 1000 (1K) individual fibers, that are thereafter oxidatively stabilized. Small tows generally contain from about 1K to 20K fibers, heavy tows contain more than 40K. The fibers or fiber tows can thereafter be formed into a knitted fabric which is then heat treated in a non-oxidizing atmosphere while the fibers are in a relaxed and unstressed condition. Heat treating the fibers increases the carbon content to form carbonaceous fibers which are substantially heat set. The fabric can then be deknitted to form non-linear fiber tows which can then be further processed, as by carding, to form a wool like fluff.
Non-linear carbonaceous fibers and the process of manufacture is disclosed in U.S. Pat. No. 4,837,076 of McCullough et al. These prior non-linear carbonaceous fibers have the disadvantage of difficult processability in forming spun yarns in that they cannot easily be formed into slivers (a continuous strand of loosely assembled fibers without twist) after carding and in subsequent drawing operations without a substantial amount of fiber breakage due to the relatively lower elongatability of the fiber. Moreover, such fibers are difficult to spin into fine yarn especially when they are blended with other synthetic or natural fibers due to the nature of their crimps. Although the crimps in the fiber are necessary for good processability, the relatively large amplitude and low frequency of the crimps in the prior art fibers causes excessive fiber breakage during carding and drawing. In addition, the prior art fibers exhibit poor cohesiveness and sacrifice elongatability to improve tenacity.
Stuffer box crimping and traditional gear crimping, which is commonly used in fiber processing, results in sharp V-type bends in the fiber wherein the outer portion of the fiber bend is subject to severe stress and the underside of the bend is subject to severe compression. These sharp bends therefore provide severely weakened portions in the fiber by causing cracking (on the outer fiber portion), creasing (on the inner fiber portion), or fibrillation. Accordingly, any defective portions of the fiber, when subjected to a bending strain, will lead to breakage at the defective portions of the fiber, especially with fibers that exhibit a greater rigidity or stiffness such as will occur in fibers that are heat treated at a higher temperature, resulting in an increase in the carbon content of the fiber.
In an article by Hall et al entitled, "Effects of Excessive Crimp on the Textile Strength and Compressive Properties of Polyester Fibers," in Journal of Applied Polymer Science, Vol 15 pp. 1539-2544 (1971), the authors describe the detrimental effects of forming sharp crimps in polyester fibers as well as other man made fibers. The authors report that excessive crimping, such as is found in V-type crimps, leads to surface damage of the fiber and a reduction in tenacity and other physical properties, e.g. elongatability which leads to fiber breakage when the fiber is placed under tension.
U.S. Pat. Nos. 4,979,274 and 4,977,654 to McCullough et al disclose apparatuses for crimping and permanently heat setting fibers without placing stress or strain on the fibers. However, the apparatuses do not produce nonlinear fibers having a reversible deflection ratio that is equal to or less than 1.2:1.
The term "reversible deflection ratio" as used herein generally applies to a helical or sinusoidal compression spring. Particular reference is made to the publication, "Mechanical Design--Theory and Practice", MacMillan Publ. Co., 1975, pp. 719 to 748: particularly Section 14-2, pages 721 to 724.
The term "permanent" or "irreversibly heat set" used herein applies to nonlinear fibers which have been heat treated under the conditions as set forth hereinafter until they possess a degree of resiliency and flexibility such that the fibers, when stretched and placed under tension to a substantially linear shape, but without exceeding the tensile strength of the fibers, will revert substantially to their original non-linear shape once the tension on the fibers is released. The foregoing terms also imply that the fibers are capable of being stretched and released over many cycles without breaking the fibers.
The term "fiber structures" herein applies to a multiplicity of filaments that are in the form of a yarn, a wool like fluff or batting, nonwoven fibers that are assembled into a web or felt, a knitted or woven cloth or fabric, or the like.
The term "crimp" as used herein refers to the waviness or nonlinearity of the fiber or fiber tow, as defined in "Man Made Fiber and Textile Dictionary" by Celanese Corporation. The term crimp includes different nonlinear configurations such as, for example, sinusoidal, coil like, and the like. In accordance with a further development of the present invention, the crimp can be a combination of two or more geometric or nongeometric configurations where one crimp is superimposed upon another crimp. For example, a complex crimp can be one in which a lower frequency crimp is superimposed upon a higher frequency crimp.
"Pseudoextensibility" refers to the elongation of a fiber, without placing the fiber under stress, in which the fiber still exhibits a residual non-linear configuration and/or false twist.
The term "uniform diameter" as used herein relates to the diameter of the fiber as drawn prior to crimping. The fiber may contain slight variations or imperfections which commonly occur during normal fiber processing operations.
The term "bending strain" of the fiber as used herein is as defined in Physical Properties of Textile Fibers., W. E. Morton and J. W. S. Hearle, The Textile Institute, Manchester, 1975, pages 407-409. The percent bending strain on a fiber is determined by the equation: EQU R=(r/R).times.100
where S is the percent (%) bending strain, r is the fiber radius, and R is the radius of curvature of bend in terms of the the crimp. That is, if the neutral plane remains in the center of the fiber, the maximum percentage tensile strain, which will be positive on the outside and negative on the inside of the bend, equals (r/R).times.100 in a circular cross-section of the fiber.
The term "carbonaceous fiber" is understood to mean that the carbon content of the polymeric precursor fiber has been increased as a result of an irreversible chemical reaction (cross linking) of the polymer during heat treatment.
The term "cohesion" or "cohesiveness" refers to the force which holds fibers together during yarn manufacture or processing. It is a function of the type and amount of lubricant used on the fiber and the fiber crimp.